The present invention relates to a method for reducing power consumption of analog receivers, in general, and to a method for reducing power consumption of an analog receiver in stand-by mode, in particular.
Cellular telephone systems are based upon communications between mobile stations, e.g. cellular telephones, and base stations. Each base station provides coverage for a geographical cell, which typically is between 3 and 24 square kilometers, and has approximately 420 channels (frequencies) available for communication, with a capacity of 40-50 conversations per cell. One of is the channels, known as the forward control channel (FOCC), is used for control communication from the base station to all of the mobile telephones in the cell. Another channel, known as the reverse control channel (RECC), is used for control communication from all of the mobile telephones in the cell to the base station. The remaining channels are used for voice communications.
A base station uses the FOCC to send a continuous wideband data stream to the mobile telephones. Messages transmitted over the FOCC are mobile telephone control messages, broadcast messages, or filler messages. An example of a mobile telephone control message is a beckoning message notifying mobile telephones in the cell that a call has come in for a particular telephone number. Mobile telephones in stand-by mode attempt to detect the beckoning message from the base station. When the mobile telephone being beckoned detects that it is being called, it uses the RECC to notify the base station. The base station then uses the FOCC to notify the mobile telephone which additional channel to use for voice communications, and the mobile telephone switches operation from stand-by to talk mode.
Reference is now made to FIG. 1, which is a schematic illustration of a format of the messages tratted over the FOCC. A message is sent in word blocks, a first word block 10 comprising a preamble 14, five repetitions A1, A2, A3, A4, A5 of a first word A, and five repetitions B1, B2, B3, B4, B5 of a second word B, interleaved with the repetitions of the word A. The base station actually transmits two messages, one composed of A-words and one composed of B-words. A given mobile telephone listens to only one of the messages (A or B), according to its unique identification number. A subsequent word block 16 comprises a preamble 14, five repetitions A1xe2x80x2, A2xe2x80x2, A3xe2x80x2, A4xe2x80x2, A5xe2x80x2 of a word Axe2x80x2, and five repetitions B1xe2x80x2, B2xe2x80x2, B3xe2x80x2, B4xe2x80x2, B5xe2x80x2 of a word Bxe2x80x2, interleaved with he repetitions of the word Axe2x80x2.
The preamble 14 comprises a standard, fixed, 10-bit dotting sequence D and a standard, fixed, 11-bit word sync sequence S, used to permit mobile telephones to synchronize with the incoming data.
Reference is now made to FIG. 2, which is a schematic illustration of a bit structure of a word, Each word contains 40 bits, formed by encoding 28 data bits with 12 parity bits according to the BCH (Bose-Chaudhuri-Rocquenghem) encoding technique, which is well known in the art and is a particular example of block coding. The 40-bit words, created by BCH encoding different sets of 28 data bits, differ by at least 5 bits. If no more than two errors are introduced into the 40-bit words, for example due to noise during transmission, then die BCH decoder can correct the errors and still accurately reproduce the 28 data bits. If there are more than two errors, the BCH decoder cannot accurately decode the 40-bit word, In this case, either the BCH decoder fails to decode the 40-bit word claiming too many errors, or a false alarm occurs, in which the BCH decoder gives the wrong 28 data bits and claims that there are no uncorrected errors.
A mobile telephone contains a BCH decoder to decode the 40-bit words received from the base station. Five repetitions of each 40-bit word are sent in order to improve the transfer reliability. A conventional technique for decoding the five repetitions of a word is majority voting, which is well known in the art. For each bit in the word, having a value of say, 0 or 1, it is always possible to choose the value that is repeated the most. The result of the majority voting is then BCH decoded. A lost message occurs when the mobile telephone fails to successfully decode the 40-bit word, even after five repetitions.
A conventional mobile telephone in stand-by mode operates its receiver continuously, receiving messages from the base station and determining whether the messages are addressed to it. It will be appreciated that keeping the receiver on continuously consumes a considerable amount of power, which is of limited supply when the mobile telephone is battery-operated.
There are a number of methods for reducing the power consumption of a receiver in stand-by mode. U.S. Pat. No. 5,140,698 to Toko describes a power saving method in a mobile telephone system. According to Toko, a receiver in stand-by mode that listens only to A-words saves power during those portions of the FOCC message composed of B-words.
U.S. Pat. No. 5,224,152 to Harte describes a different power saving arrangement and method in a portable cellular telephone system. According to Harte, a receiver in stand-by mode decodes each word as it is received. Referring back to FIG. 1, if the BCH decoder successfully decodes A1, and the mobile telephone determines that the A-word is not addressed to it, then the time T1 remaining until the next dotting signal D is calculated, and the power to the receiver is reduced during the time period T1. If the BCH decoder does not successfully decode A1, then the receiver remains at full power. If the BCH decoder successfully decodes A2 when it is received, and the mobile telephone determines that the A-word is not addressed to it, then the time T2 remaining until the next dotting signal D is calculated, and the power to the receiver is reduced during the time period T2.
It will be appreciated that according to Harte, once a repetition of a 40-bit word is not successfully decoded, the information contained therein is not used further to decode subsequent repetitions.
It will also be appreciated by those skilled in the art that when the FOCC is relatively noisy, the device disclosed by Harte will spend relatively little time in the low-power state. More importantly, it will suffer from a higher rate of lost messages than conventional mobile telephones that do majority voting.
U.S. Pat. No. 5,175,874 to Auchter describes a power saving method in which the receiver reduces power as soon as two copies of a word have been successfully decoded and are identical. It will be appreciated that if the FOCC has a low noise level, then a device according to Auchter will remain at full power for longer periods of time than a device according to Harte.
Another method for reducing the power consumption of a receiver in stand-by mode is described in U.S. Pat. No. 5,568,513 to Croft et al. Croft uses soft bits, which are decision variables containing information about the quality of the bits. In conventional analog mobile telephones, the analog signal received by the telephone receiver is filtered, converted to a digital signal, and then filtered by a Manchester demodulator, resulting in soft bits, The soft bits are quantized, and the resulting hard bits (having one of two values, say 0 and 1) are decoded Referring back to FIG. 1, Croft discloses a method whereby the soft bits of A1 are quantized, and then the hard bits of word A1 are BCH decoded. Croft calculates the syndrome of the 40-bit word A1, using a well-known technique similar to that described in Lin, Costello, Error Control Coding Fundamentals and Applications, Prentice Hall (1983), pp. 58-60. The syndrome indicates whether the 40 bits are error free, or whether there is a single error, or whether there is more than a single error. In the case of a single error, the syndrome also indicates in which of the 40 bits the error occurs, and Croft corrects the error. Croft refers to the error-free state or the corrected-single-error state as a successful cyclic redundancy check (CRC), and refers to the multiple-errors state as an unsuccessful CRC. If the CRC of A1 is successful, and the word A is not addressed to the mobile telephone, then the power to the receiver is reduced until the next dating and sync sequence is expected If the CRC of A1 indicates that there are uncorrected errors, then the soft bits of A2 are accumulated with the soft bits of A1, the sum is quantized, and the resulting hard bits are BCH decoded. If the CRC of the result is successful, and the word A is not addressed to the mobile telephone, then the power to the receiver is reduced until the new dotting and sync sequence is expected.
Croft teaches a cumulative majority vote in which soft bits are added together. Given a particular signal-to-noise ratio (SNR), a device according to Croft reduces power to the receiver for longer periods of time and has fewer lost messages than a device according to Harte.
U.S. Pat. No. 5,406,613 to Peponides et al. describes yet another power saving method and apparatus for cellular telephones. Like Croft, Peponides teaches adding soft bits together prior to BCH decoding. However, Peponides also uses the VSP (variance of signal power), which is a measure of the noise in the signal, to determine whether to accept a received word. If there is too much noise, the word is rejected. If the noise is acceptable, then the word is soft combined with previous words whose noise is acceptable. Soft bits in the combination whose amplitudes are too small are declared erasures, and if the word has too many erasures, then the next word is received as well. If the number of erasures is acceptable, then the soft combination is quantized and BCH decoded.